Help with electrical nightmare

I really really really would be indebted to anyone who could help me through this:

I have a 1991 BMW 325i automatic which has had electrical issues for awhile. Quite routinely the car will not start. The sounds it makes is like the starter is not receiving enough current to turn the flywheel. So far I have had the starter replaced, the ignition switch and lock tumbler replaced, new battery. I've removed the alarm system because initially we thought it was a killswitch issue. When I tap the solenoid to the starter to send current directly to the starter it gets the current it needs and engine turns. But the starter and solenoid are new and the same thing happened with the old starter. I don't know what else it could be. A relay perhaps? I can't find schematics for electrical and the dealer already ate through quite a bit of money replacing things that probably did not need replacing.

Please help..

Thank you so much

It's a body ground check the oilpan and make sure that ground is connected and check other grounds as well that happened to me and I tried "jumping" it and it would try to turn over but it was just a ground wire also check ur fuses and relays.

Firstly there is the switch on your auto box that prevents starting unless you are in park or neutral...this switch could be faulty or needs adjustment.
Then there are the "unloader relays" K5 and K7...these relays have to drop out when you start the engine. These relays normally supply power to the air conditioner, heated seats etc. and they need to drop power to these components at the time the starter motor is energised. There should be a black/green wire going to the solenoid. When the solenoid is activated, this black/green wire makes contact with the large positive lead that comes from the battery. This black/green wire then goes to relays K5 and K7 and supplies + 12 volts to the other leg of the relays.....+12 volts on both sides of the relays causes the relays to drop out. If you are not sure where this black/green wire is supposed to connect on the solenoid...some solenoids are different to others....fit a large lug on the black/green wire and connect it to the large lug on the solenoid....not the one that has +12 volts all the time from the battery...the other lug that gets the +12 volts switched to it when the solenoid closes. You should now see relays K5 and K7 (you may only have one fitted) drop out when the starter is activated and then re-energise so that your air-con etc. can now work.
Also check that you have a very good clean, tight earth strap earthing the engine to the chassis .

Edit to add: www.e30tech.com has a good FAQs & DIY section that covers issues like this.

Almost always a ground.