I seem to recall that the heating elements slide into the pockets/rolls/pleats of the seat, and from there it's a wiring thing. If the design of the seat pleats (I'm looking for words to describe the pattern of upholstery padding here) is the same or similar, you should be able to retro-fit the elements from your 500SEL into the 300SE seats. A lot of work though, and hardly worth it unless you really use the heating function.
