congrats on your purchase, my santro reverse gear light also not working, i have checked everything, even replaced the reverse gear switch on gear box, but of no use, the mechanic advised to open the gearbox for that, i have not gone for that option, don't buy reverse gear switch, rather check with the existing one, that will be fine but the reverse gear doesn't kick it in or out (may be your case is same)
for sensor and electrical works, try to do the jobs from authorized dealers, all other mechanical miner works can be done with ordinary road side mechanics.