I know this one!!!!!
Do a search for cruise control amplifier.. You will find more info than you will ever need. It is highly likely that that is the cause of your problem.. It's very common..
89 300SEL Money Pit
92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week
88 Wagoneer Slightly modified (Not for soccer moms)
04 Kia Sedona with every option... NICE