The factory location for the two seat heater switches are below the ashtray, on the panel between the gear lever console and the ashtray.

You can get upper console panels (with the row of switches) in a variety of configurations. You can get up to and including the FULL ROW of cutouts for the switches. Unfortunately a new piece of wood runs between $500 and $600.

if you can find a piece of wood from an early 500E (1992 or 1993 model) it will have all cutouts except two, and the 1994 models have all cutouts except one. The piece of wood with the full row of cutouts I believe was used in the cabriolets.

Also I don't know if these pieces of wood with the higher number of cutouts were available in Zebrano wood (used in normal 124s) or burl walnut which was used in the coupe, cabrio and V8 cars. This would be worth checking out with a dealer.

I think your best bet is to purchase the smaller piece of wood (or getting it from a wrecking yard) with the two switch cutouts where the heating switches were originally intended to be. these would be far more common than the upper pieces with the higher number of cutouts.

Good luck!

Cheers,
Gerry