Since the OP isn't making or selling these any more I thought I'd throw up another option.
This only works if you have the trailer harness up by the spare tire or plugged into the back of the 7 way plug:
Buy this: CURT 56584 Custom Wiring Harness
https://www.amazon.com/dp/B007EA6WQU/ref=cm_sw_r_cp_apa_i_JHO5AbWPJPPR0
Clip off the 5 pin plug and add two of these with associated pins and seals with some wire. You get a ground, reverse, brake/turn L/R and park.
https://m.delcity.net/store/GT-150--Male-Housing/p_810485.h_810490
Plug it in and go. Should cost less than $50 for everything brand new.
Only thing you would need to do is add a resistor to keep the blinkers from hyperflash.
I can assemble this for you if you don't like wiring stuff for a fee. Just thought I'd try to help whoever is still looking to do the mod.