Randy, in two of our cars the switch has been the culprit in the automatic. Best approach is tracing with a test light:

First stop, Fuse 12, blue/yellow lead outbound.
Next, black wire at the left taillight housing, backprobe the actual connector terminal.
If needed, remove the cover on the shifter. The feed is a yellow wire on the switch. The switched output is the black wire on the switch. Here's how I fixed the switch in two cars so far: Fixing your backup lights for nothing
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Art Benstein near Baltimore