SO... you think they put ABS on every 91-up XJ/MJ sold in Canada just so the DRL's would work? Because they were indeed mandatory from '91 on. You probably didn't realize that for the '91 model year the speedo's changed from mechanical to electronic, thus the signal to activate the lights simply comes from the same speed sensor that feeds the speedo. That being said the ones on my '91 never worked (and I never cared), but I have a '93 here scrapped that they did work properly. We've also had a '91 US model that was rigged up so that as soon as you stepped on the brake to shift from park it activated the park light circuit. I kind of liked the idea that it put on lights in both the front and rear, but as it also activated the dash lights it was hard to remember to turn on the headlights when it started getting dark. We also have a '90 US model XJ here that for some reason also has DRL's. Anyone know if that might have been an option on the Limited's? They aren't speed sensitive and activate the high beams at reduced voltage when the key is in the run position. I haven't looked real closely at the headlight wiring on that rig but it didn't appear to be a hack job.
Anyway I would guess that the original posters rig, being a '91, would have all the required wiring in place so it should be as easy a sourcing the module and plug 'n play. Good luck and if you want I can look a little closer at the '90 and see if that system could be easily (and neatly) adapted.