This article is from the Watercooled VWs General FAQ, by Jan Vandenbrande with numerous contributions by others.
A: Most likely your back up light switch in the transmission is broken. In
some cars it will also disable the upshift indicator. According to
rgolen@smucs1.umassd.edu, short shift kits reduce the longevity of these
switches to 3-4 months.
 
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