@nounours You'll be looking for continuity from the traces on the cart pcb. You'll also have to desolder chips or remove them from the circuit by scratching out the traces or they'll interfere. You need a clear trace or the USB stick will be hard to read.
Look at the pic of my cart. Traces go from the front of the cart to the back and to the front again. Try and find the 4 shortest traces for less interference. Thats what the multimeter will do to make your life easier. If it beeps from one end to the other then it should work.
There are solder points very near to the cart pins too. You can use those but you must use the same pins for all the carts, of course. Depending on the cart, those might be the best to use. Here is an example. 4 short traces. Then the traces are scratched out from the back out so they're cut off from the rest of the circuit for less interefence.
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