I'd suggest a two prong attack on the problem.

One, take the car to an auto parts store with a tester and have them run a free electrical test. Their testers are pretty good at finding out the cause of your problems and best of all, the test is free. Also, check the obvious like a burnt out fuse. Relays are also notorious and need to be checked. The starter itself may also be bad and in need of replacement, check other threads for how to check the solonoid and the internals of the starter. I once replaced the starter on the 1984 300DT with a Beck Arnley brand starter TWICE. I finally ended up buying a genuine Bosch starter which solved my starting problems. Again, test before replacing because throwing parts at a car can get expensive.

Second, if all this doesn't work, the next step is an auto electrical repair shop. The competency of these types of shops varies, so be sure and check out their BBB and online reviews. If they know their stuff, they should be able to find the problem in under two hours.

I've seen Merceces Benz trained mechanics spend hours trying to track down electrical issues. German electrics are different than domestic electrics. Sometimes the problem appears to be obvious, othertimes it takes a real automotive electrical expert to hunt down the problem. You can spend HOURS of your own time trying to track down electrical problems. Paying a competent electrical shop willl save you hours in frustration (ask me how I know).

Just my two cents......