I have a 1993 300D with the same symptoms...ABS light On, no Tach and no A/C. I tried cleaning all the contacts on the OVP relay and while the problem got better, it was not cured. Today I peeled away the aluminum casing of the relay and resoldered all the connections. I wrapped the whole relay with electrical tape to keep the moisture out and plugged it back in. So far it looks like it worked. I also scraped down the fuse contacts on the inside and they did have quite a bit of corrosion on them.