I would recommend you purchase a service manual and electrical troubleshooting manual. You can buy these as a set online for about $75. It's worth the money to have all the recipes for working on your GW and what and where all those wires and connections are.
Welcome pedro66. I was in the same position the last part of last year, looking for a way to intergrate my iPod with the GL1800 controls. I did the iremote and didn't like it because I had to handle a remote to change a song while riding. I finally discovered a unit made by Dension, some company in europe. The device is called Gateway 100. I bought it through another company named Mass Electronics. After it was all said and done the cost was under $300.00. That may sound like alot just to listen to your iPod on your bike but, being able to control the iPod with the OEM cd controls already on the bike was well worth it. Lets face it there is nothing concerning the Goldwing that is cheap, except learning to do some of your on work.
To see the product go to dension.com select your iPod type then Gateway 100, then under compatibility select Honda, then you'll see Goldwing listed. You'll have to contact Mass Electronics at (905)764-9533, to purchase what you'll need. These are the part #s I purchased GW16HB1 and IPO2CR9. My experience with them was great and they stand behind the product they sell. I hope this helps and save you some time.