This stuff gets confusing. Part of the confusion is terminology. In an RV a/c, both the fan and the compressor require a RUN capacitor. One each, but they may be separate or two sections of a combined capacitor. Usually in a metal "can" but some are enclosed in plastic. Fan and compressor will not start without those RUN capacitors. By without I mean either missing or defective. But the fact that a RUN capacitor allows a fan or compressor to start does not make that RUN capacitor a START capacitor.
As OP found out, a START capacitor piggybacks onto the compressor RUN capacitor. It's an addition, nothing gets removed. But the START capacitor does get removed, electrically, once the compressor gets up to speed. Think of cranking your car. You take the Key out of Start and leave it in RUN once the engine has started. So there's a START capacitor that's cylindrical plastic with a retaining ring at the top holding it together. The "Key" that takes it out of circuit is a PTCR, a Relay, or some form of Electronics. START capacitor and PTCR connect in series, one after the other, and one PTCR terminal goes to one compressor RUN capacitor terminal. Other PTCR terminal goes to one terminal of START capacitor. Other START capacitor terminal goes to the other RUN capacitor terminal. SPP6/SPP6e have the PTCR or Electronics factory-attached to that START capacitor, so it's just two wires, and they go to the two compressor RUN capacitor terminals. Either way, no Polarity involved.
The Relay type START kits, and some Electronic ones, may have another wire. You have to follow the a/c wiring diagram if it's OEM, or the supplier's diagram if it's an aftermarket item.
Just remember: At least TWO RUN capacitors. One or two physical items. If there's a Third capacitor it's RUN, compressor only, across compressor terminals on its RUN capacitor, has to have PTCR or some other disconnect-after-start device.
