I recently installed a mp3 ecu in my p5. It's pretty easy. Basically, it's under the front passengers feet.
You remove the front passenger doors side sill, (just pull up), then the trim down on the right and left hand side of where the passengers feet would be. (It has those little clips that hold the trim in place, kinda like dry wall plugs)
Then you'll be looking at a big silver plate that has four nuts that need removed.(after pulling back the carpet of course) Once those are removed you can bend it back to expose where the ecm chip is. It helps if you have one in hand so you can recognize what your looking at. Anyhow, theres a bunch of wires going into it.
Theres a pdf with the actual procedure floating around, I'll look for it. One note though, the (plug/interface wire) part that connects to the mp3 ecu pins is bolted in place, so it's not just a matter of pulling hard on the wiring bundle..., it's bolted in making sure it's seated properly and wont jiggle loose while driving etc.
And while it makes perfect sense, I didnt see it at first. And it's not indicated on the pdf i was using.
Good luck! I like the MP3 upgrade on my p5!
Sorry if it's too vague. I'll look for the diagram.
-jason
Edit : clipped this from the pdf. I didnt have to screw with "part 4/brackets" shown in the diagram.
