Okay, does this happen all of the time, or just some of the time?
If it's all of the time, then there is a connection between the radio and the transmission range selector switch.
I only have the wiring diagrams for gen II, so gen I may be a little different.
There is a wire going to the radio that is hot only when the starter is engaged. It's purpose is to cut the radio off while you are starting the car. On gen II, it is gray with an orange band. This same signal goes to the transmission switch as part of the starter lock out when in gear. If the transmission switch is messed up, it might backfeed voltage on this line. On the gen II, you can pull fuse F2.02 to cut the signal to the radio and find out if this is the problem.