Finished the install on my Accel 375+ unit tonight. Have been running the 300+ system with no problems. Here is what I did on the flat 6-pin wiring:

red: 12V switched
black: grounded to the same place on the frame as the 300+
yellow: hot side (when toggle switch is on) of the WOT switch to
activate timing retard function
purple: FPSS for ground
orange/white: not used

When the 375+ 4-pin connectors are hooked to the 300+ box and car's wiring harness, it will not start. When disconnected and just run off the 300+, it starts fine. The 375+ has power because the keypad lights up and can be programmed. Any suggestions? Thanks.