Hi,
Unable to find a copy of the original code, I got hold of a GPL disassembler to produce a disassembly of the original GROM.
The one I used was xdg99 (https://github.com/endlos99/xdt99/blob/master/xdg99.py) which worked a treat.
Then I trawled my way throught the GPL docs here : http://www.unige.ch/medecine/nouspikel/ti99/gpl.htm
Finally, I managed to track down the routines for handling crashing into the opponent car, modified the GROM binary and used Classic99 emulator to run the modified cartridge GROM.
The results... I can play Car Wars without colliding with enemy cars to see what the later levels are like!
Video here :
If anyone wants to do the same, there are 3 bytes in the CarWarsG.BIN to patch.
Change the bytes at 0d52, 0d8c and 0d6f to zero.
Enjoy