.....I remember when shopping for a car once, saw one, a Mitsubishi
Diamante I think, that had a line inputs jack on the front panel of
the radio.....................

Hmmm, curious, what kind of jack was it, the standard little stereo
headphone jack? How did you select the input. I've been thinking
about making a mod like this for my car. My guess is to simply wire a
standard little jack across the radio volumne conrol then drive in from
the low Z headphone output of the portable which will "override"
whatever the radio is trying to play.

Mark