How does one find out why

[e30itis]

Fuse #17 which controls the power window switches on my 92 318ic has just blown have gotten it replaced but how does one find out why it is blowing/Has blown?

[Beeker1972]

If it blows again that is telling you that something is bad in the circuit  ... one of the switches, motors, or could be the window mechanisms needs to be cleaned, and lubricated.   

[DesktopDave]

Try to break the system down to a failing component. Did the breaker pop at all? The power windows are unique on an E30 - they have both a fuse and a breaker. The breaker is usually mounted in the center panel along with the defrost and 4-way switches.

Do you have the ETM?

There are only switches and window motors on that circuit. I'd isolate the circuit by unplugging the window switches first. Plug one in, put the window up & down, see if you can burn out the fuse. If not, put the other switches in & repeat until you find the bad section in the circuit. Once you've done that, swap a known good switch to determine if it's the motor or the switch.