We have a 190E that we are using for chump car and the key broke off in the ignition so we decided to go with a toggle and push button. I marked things so I know what colors go to what pins on the tumbler and I've found which one turns on the fuel pump and the one to make it crank but I can't get it to stay running. This car is an '89. Did Mercedes have anti theft systems at that time? What could I possibly be doing wrong that's keeping it from running?

You are just missing the wire that have constant 12v, check the wire diagram and you will see what happens. No anti theft at this car.

You need a wiring diagram. It isn't as simple as adding a single switch.

You might have to use a double pole or triple pole sw ti get things going. If you tie everything together in a single pole sw, the car might not shut off due to back feeding.

In general, there is an engine run circuit , accessory circuit < ( sometimes 2 ) , maybe a charging trigger circuit.