Start with the basics and go from there. Read through as much info here as you can and educate yourself. Check all connections, fuses, and relays.
Take it to a mechanic when you are thoroughly stumped, and refuse to accept shotgun diagnostics.
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.
1973 220 (gas)