BMW: No start. Aftermarket items?, aftermarket radio, aftermarket stereo

Question
Hi Josh,

My "new" 87 325i isn't starting now.  I just had an aftermarket radio installed and an aftermarket alarm.  The car started fine for about 2 or 3 times after those items went in during the span of a week or so.  Now, the car won't start at all. I turn the key and nothing happens except lights come on in the dash (check engine, etc.).  So I'm not sure if it's the battery, the code, or maybe the starter went out, or something else.  I say the code because when I bought the car, I had it smog checked and the must have disconnected the battery.  When I got the car back, it asked for a code, which I didn't have.  When I got the car alarm installed, I went ahead and just had a new CD player installed so I didn't bother getting the code and setting it.  Is it possible for the code to disengage the starter somehow?  I was reading in some manual about setting the code and something about the starter motor will turn but the engine will not start, but that didn't make sense.  If that is the problem, I'm not sure what to do because I have an aftermarket stereo in now.  I wouldn't know how to enter the code unless I could disable it.  I'm hoping it's something else.  What do you think?

Also, when it was running, all of a sudden the car started to rev the rpm's by itself when I was coming to stops.  The rpm would dip down then rev up to about 2000 rpm then back very low, then back to 2000.  It would do this about 3 times, then settle around 800 rpm.  Once before that, it just continued to do this, more than just the 3 times.  I would barely touch the gas pedal and it would stop doing it and settle at around 800 rpm again.  What is causing this?

Thank you very much.

Answer
Tom,
 I doubt the alarm is the problem, since the car was running funny, it presents a clear picture that there is something mechanically wrong with the car.
 Instead of looking at what "might" have caused this problem, lets narrow down to what we KNOW is the problem.

 DO you have spark???
 
 Do you have Fuel???

 Either of those will cause the engine not to run.  Since your starter is working it HAS to be fuel or spark that is keeping the engine from starting.

 Find out where your problem is and you'll be on your way to fixing the car.
 
 Good luck,
 Josh