I believe the original location for the installed stock horn on my 88 is in the left rear quarter in the engine compartment against the side. It appears that part of the black plastic cover was cut away to allow the horn to fit.
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Randall
1988 911 Cabriolet (SW Chip)
2000 Honda Civic (4dr wifey mobile)
2001 Honda S2000 (daily driver)
2003 Honda Odyssey (family truckster)
1978 911 SC coupe (Sold)
My 911 Cab Pics