long story, but worth the read in my opinion. my dad always taught me that if you learn something you should share it because no one else wants to go through what you did if they could have known before it happened.
i bought a pioneer head unit, steering wheel interface, and data bus interface (for the door chimes, warnings, etc.) all on ebay.
got the head unit running in the car no problem. the rca cables were connecting to the back of the head unit, and the music was all distorted and sounded terrible. also, when i shut the doors, the stereo turned on and made horrible noises through the speakers (wtf).
after spending 2 days troubleshooting on and off, my dad finally forced me to go to circuit city and said he would pay for the install. because i am only home for 3 weeks on vacation from work.
circuit city notified me that my interface was incorrect, and luckily since i got the brand that they carry, they swapped it out free of charge. also, since i got a stereo that they carry, they installed it free too. the only thing they couldn't refund was the steering wheel control interface. i had to buy their brand and the install for that. all in all, it ran me about 130, but i will resell the steering wheel interface on ebay or if someone is interested on here, i will sell cheaper.
bottom line, if you are installing an aftermarket head unit and you have an 04+ GP without the monsoon system, you need the GMOS-01 data bus interface.
very good piece of info in my opinion...anyone that has questions about it, i will be happy to answer to the best of my knowledge, because i basically did the full install, but couldn't complete the job because of the incorrect data bus interface.