great choice. Now that you mention its a 7894 though, Alpine reccomends that this HU be connected directly to the battery like a real amplifier due its 240W output. It doesn't have to be, so it's up to you if you want to run a power wire. I'll be honest, mine is not, and it still sounds great. As said, to wire the alpine harness to the adpater harness all you need to do is match up colors (and double check the wording on the wires matches) and just plug them all together. then you just need to remove the factory HU, install the sleeve and secure it in place, install the alpine and trim ring and you're done.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>