Added a 3.5mm jack to the radio yesterday. Spliced it into the CD changer harness, spent about $15 total. I'm thinking the next time I do it, I could probably do it through the normal radio harness "out" connector. Used the guide from the FAQ and wjjeeps.com for the harness since the post in the FAQ didn't apply to my '99. Going by the post in the FAQ, it keeps the CD changer playing a CD.
1999 WJ Limited 4.7 Quadra-drive
18" JK Sahara's, 245/55/18 BFG T/A Spec's, black headlight housings, cherry bomb pro muffler, ebay intake, Sonnax LB2, Transgo HD2, de-roof racked and de-badged.