Can you trace it to a pin? There are codes on each pin that tell you to what it connects. The 87 pin (opposite the blank) connects to a solder joint about in the bottom middle of the board. The 87 pin connects to the compressor supporting Peter's hypothesis. The 15 pin beside the 87 pin that connects to the board beside the 87 connection is the main power supply to the relay.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD