Firstly you have to find it out, whether it is a signal problem or a power problem.
Concerning touchscreen: I suppose it is VGA, since HR mons are S-Video...
Run HR monitors from another battery. Is the problem persists?
If not, use ferrite core coils and filtering capacitors at power wires.
If yes, the root of the problem is in signaling. Try to cut the ground wire of the cable. Try to reduce the lenght of cable as short as possible (use a very short test cable). Use double-shielded cable.
What happenned if you use just one HR mon?
Try to provide separate THICK! power/GND cables for each HR mon.
Use 'hard' grounding, I mean use the parts of the metal chassis of your car as GND.