Factory harness plugs into the EZ, then the EZ harness goes into the TST, then TST to engine. With that SP66 I would start off with the EZ on level 2 then 4 and 1 on the TST. I'm sure others will chime in also. I'm running mine right now EZ-2, TST 3 and 1 even pulling my big trailer around. My . 02