Brake assist was first used by Mercedes in 1996. It senses pedal pressure and applies the brakes more forcefully in a panic stop. I don't think your '89 has brake assist.
You mention plugs at the hitch. Sounds like an electric brake controller for a toad or trailer. Much more details needed to help. Brand? Serial number? Photos?
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Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI
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