Mercedes cruise control ideas

According to BenzWorld, the cruise control not working could be caused by a faulty ABS speed sensor (not a cheap replacement — about $110 per wheel). It could also be caused by tire tread wear being different enough among the wheels to throw off the computer’s calculations, disengaging the cruise control. Or it could be something else, which might be indicated by stored codes.

This page has instructions for using the supposedly built-in blinker box. I’m not sure if it’s present on our 300SD. If not, that same page has instructions for how to build a blinker box.