Maybe I'm unclear about the set-up, but if there were presently no ground the lights wouldn't work at all. Maybe look into one of these. I just pulled up one from Google. You can probably find it elsewhere. I hear Badland's version works well. I also hear to stay away from Biketronics version, but that was three years ago so maybe they've fixed their problem:
From Badlands site:
"On 2000 and up the load equalizer III is a must. It was designed for the new breed of turn signal (TSM) and turn signal security system (TSSM) modules. On TSSM models, a load equalizer I & II will cause erratic flashing and the key icon to flash and not reset to the correct code in the TSSM module. This will shorten the life of the TSSM security module, unless you are using the load equalizer III.
On aftermarket alarm systems, part of the security is for the 4 ways / hazards to flash when the alarm has been tripped. The load equalizer I will melt after an extended period of time (per the instructions) and the load equalizer II will eventually crack over years of use. My recommendation is the load equalizer III due to its solid state construction that does not use ceramic resistors.
The H-D turn signal converter, turns the rear turn signals into brake lights while maintaining turn signal function. For stock application this module works fine. However, if custom turn signals are installed and you use a load equalizer i or II, the turn signal wire now becomes live when the brake is applied. Now the load equalizer is building heat every time the brake is applied, and during the turn signal mode. My recommendation would be, to use the Load equalizer III or the “illuminator module”. Both have a built in solid state load equalizer that will not get hot, and adds the running light feature, as well as the brake light feature."