Zoso's excellent radio thread, copied from his old page. Please let me know if this isn't kosher (I'll take them down), but these how-to's are so good I had to re-post them here.
318i Modification -
CD/MP3 Player Installation When I purchased the car the radio was missing. The previous owner had a system in the car and ripped it out before selling the car. Hanging out of the dash were several sets of RCA cables and numerous random wires. Whatever factory radio harness used to be in the car was gone. It took a while to discover what the previous owner had done to the car. Luckily the guy left me with a good starting ground for installing my new system. Like I said, the RCAs were already running to the trunk along with a amp turn on line. The 5.25" speakers in the rear deck lid and the front kick plates were upgraded with Kicker and Kenwood speakers. Pod style tweeters were added to the plastic plates that are near where the side view mirrors attach and crossovers were installed to the front speakers and the tweeters. All the speakers were wired into the trunk. There were already thick power and ground wires leading to the trunk.
I have a 4 channel, 200W Denon amplifier that I wanted to use in the car. The next step was finding a suitable head unit. My goals for the head unit were:
1) Needs to play CDs and MP3s on CD-R and CD-RW formats. I especially wanted MP3s because I have a large collection and it is very handy to be able to put them onto one CD and have a ton of music available. I also have several audio books and "learn foreign language" audio books that I'd love to listen to on my hour long commute to work. I wanted the ability to play CD-RWs because if I burn an audio book and listen to it, I want to be able to reuse the disc to burn another audio book.
2) Needs to have a simple, flat black faceplate.My M3 has a Kenwood fold out screen with NAV and DVD. I love the system, but when the screen is closed, it does not match the interior of the car. I really wanted something that would look good in the car. Too many head units I saw in my search had silver or glossy black faceplates with crazy looking buttons and graphics on the LCD screens. Most of them are obnoxious looking.
3) Needs to have red buttons and screen.I really wanted the control buttons and screen to match the dash lights in the car. I definitely didn't want a blue screen or green control buttons.
My search finalized with the
Clarion DXZ645MP It has a relatively simple faceplate and is flat black. The great part about this head unit is that you can change the colors of both the LCD display and the control buttons to be between 80 different colors. It also satisfies my CD/MP3 capability with the ability to play CD-RWs.
I bought it from
Audio Warehouse Express. I saw it for less money on Ebay but there were several reasons why I got it from AWE. First, the Ebay sellers were playing games with the numbers. There were charging $25 for shipping and then $5 for insurance. They also were adding 7% to 9% state sales tax and they were all out of state. That sales tax was going right into their pocket. With these extra fees, the Ebay price came within the $20 of the AWE price. The Ebay sellers supposedly would replace defective equipment but it had to be within 3 days of receiving the product and a certified stereo repair shop had to take the defective radio and prove that it was defective when you received it. Can you see where this is going? The Ebay seller's warranty is crap. I don't want to be rushed to installing the unit to see if it works and I don't want to have to take it to a stereo repair shop and pay for them to troubleshoot it. The Clarion comes with a 1 year factory warranty, but only if you buy it from a certified dealer. The certified dealers sell the unit for a good amount more than a non-certified dealer. AWE isn't a certified dealer but they give a replacement warranty that isn't absurd like that of the Ebay seller's. I ended up paying about $10 for shipping and got it the very next day.
BMW Parts:Antenna Grommet- 65 21 1 376 008
Click on the thumbnails for larger pictures.

Here is a picture of my car when I was removing the dash cluster to replace some dash console lights. When I removed the lower knee bolster below the steering wheel, a pile of wires fell out. The wiring was a mess. The previous owner had added a mile of wiring. I suspect there is the remnants of some stereo modifications and an alarm system (notice the square shock sensor below the dash cluster?). There were wires with metal exposed. He had a wire connected to a splice, connected to another splice, connected to yet another splice. It took a while, but this rats nest of wires was removed.

While digging behind the back seat, I found this device. I posted a picture on Roadfly.Org (bimmer.org) asking what it is. The friendly forum members told me that it had to be a bomb or a weapon of mass destruction. After taking it out, it is my best guess that it was an old Lo-Jack system. The orange part on the bottom is a battery and the metal box near the top had coax coming out to it going to an antenna. Perhaps it was one of the first Lo-Jack systems because it is so big and so poorly hidden.

Some E30s were equipped with a hole going to the trunk for a ski-bag. My 318i wasn't equipped with the hole, but the metal is easily popped out. One tap with a large hammer cohersed the metal out.

The plan is to mount my 12" Kicker Subwoofer to a plank of wood and attach it to the inside of the trunk so the sub points toward the hole behind the rear seat. Start with two thick pieces of MDF. Attach some glue.

Glue and screw the second piece of MDF to the first.

After the glue dries, measure the trunk and begin to trim the wood.

Cut out a hole for the sub.

The trace is where the ski-hole is.

This is the finished shape. Spray paint the MDF or cover it with carpet.

Run some RCA wires from the stereo to the trunk. From what I have seen and read, there seems to be less crosstalk problems if you run the wires down the driver's side of the car.

The rear trunk metal was lined with Accumat.

The new sub deck is mounted to the metal in the trunk by drilling holes through the metal and wood and using screws, washers, and bolts.

I purchased a 3/4/5/6 channel Alpine 3566 amplifier from eBay. A bit more Dynamat is used under where the amp will sit to help with vibrations.

Here is a picture of the amp attached.

From inside the car, here is the sub peeking through the ski-hole.

Same picture with the padding back in place.

The left over Accumat was used around the front speakers.

I added the factory premium sound system tweeter pods. They look great.

Another picture of the tweeter pods.

Here is a picture of my antenna motor and mast. I like how the previous owner supported it with a chain of zip-ties.
The connector has three wires going to it. Red - 12V, Black - GND, White - On/Off. I couldn't find the stock on/off wire near the dash so I simply cut the wire and attached it to the amplifier turn on from the radio. Works perfectly. While you are back there, replace your antenna grommet so water doesn't get into your trunk. I used a little black gasket sealant between the grommet and the antenna mast hold to make a better seal.