No power when connected to car

Hello: I wanted to listen to my MP3 player (no, it's not an iPod) in my car without using an FM modulator, so I found a cable that plugs into the CD-changer connection (IP-bus) on the back of my Pioneer car radio and has L/R RCA connections on the other end. The idea is to play music from the MP3 player and the stereo will find hear a signal when switching inputs (or Sources, as Pioneer calls them). Anyway, it was hard for me to reach the back of the stereo even after taking the panels off, so I decided that it'd be easier with the head unit removed. I didn't have the instruments to pull out the head unit, so I took it to a car stereo installer who pulled it out free of charge, as long as I did my work off premises. So he takes out the unit, I plug in the special cable on the back and all of a sudden the car stero won't turn on. I didn't think it was the installer's fault because all he did was pull out the head unit a little bit and I finished taking it all the way out. I got home and was scratching my head as to why it didn't work. A friend of mine loaned me a power supply unit (PSU) that I set to 12 volts. I was able to turn the stereo on by grounding the negative from the PSU and applying power to the red (AC?) and yellow (continuous?) on the radio. I'm able to turn it on applying power directly to the pins on the back of the unit and also from the beginning of the wiring harness when connected to the stereo, so I have power continuity all the way. That ruled out anything wrong with the radio, and everyone else says that of course there's something wrong with the power on the car. I used a Fluke to test voltage on the power connection coming from the car but honestly I wasn't sure if the voltage reading was correct or if it changed too much by me moving the Fluke's pins. Anyway, there was some reading, not zeroes. What should I be getting on the Fluke? I tried spare fuses, but unless the spares are bad, there's something else going on. Just so you know, I do have an amplifier and subwoofer in the back of the vehicle. There is a cable coming from the amplifier that connects directly to a wire on the Pioneer's side of the wiring harness. This enables the car stereo to turn on the amplifier. I tested the car stereo without the amplifier wire connected just to see, but nothing. Another thing, I haven't pushed the head unit all the way in because that would mean another trip to the installer to remove it if it doesn't work for more troubleshooting. I hope it doesn't have to be pushed all the way in for it to work... :-( Do you guys have any troubleshooting suggestions? A friend of mine mentioned something called "security codes", but I don't think that applies to aftermarket units, does it? Please advise. Thanks.

Aubreyq: It will help us to know what kind of vehicle, make model, and year your car is, as well as what kind of Pioneer HU, ( model #). As far as what your Fluke should read, it should read 12V if there is power going to it. The +12V memory (yellow) should read 12V when the car is off and the Switched +12V (red) should read 12V when the key is in the accessory position. Hope that helps some.

Sorry, I can't believe I forgot those details.  It's a '93 Honda Civic DX Coupe.  The radio is a Pioneer DEH-P4100.

Did your Multimeter give you positive results then, your car did indeed register 12V off and 12V in ACC? Silly question but does the radio work without the IP-Bus cable plugged in? The only other thing I can suggest is making sure that the radio is grounded properly in the car. If you are using the harness ground with no luck, try physically grounding the unit to a metal part of the chassis.

you should have received a set of keys to remove the radio when you purchased it.  If you don't have them ask the installer; I know we've got at least a million of those little bastards lying around the shop.  I know some installers will keep them basically to prevent you from messing with their install, but if you're doing the work yourself you should have them.  It will save you the trip to the installer should you ever need to remove the radio again.  Oh and some afternarket headunits do run with a security code that will lock the unit if power is removed from it, but Pioneer does not have this annoyance.Life moves pretty fast; if you don't stop and look around once in a while; you could miss it.