I looked in the ETM for a 123 and it appears they were offered as an option.

Fuse 4 is the power source (live in I, II, or III)

The switches are arranged so that the seat pads (backrest pad and cushion pad) are connected in series for low heat and in parallel for high heat.

I found the wiring diagram online and it's pretty straightforward. No relays or control boxes like in the later models.

Later models may be harder to integrate with those switches, as the later models use a relay or controller to accept momentary switch inputs, whereas on the 123 the switches were maintained.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech