If the rear wiper doesn't come on when selecting reverse then its more than likely the reverse sensor switch on the gearbox. I assume you've checked the obvious stuff, bulbs, earths, fuses etc. Its a pretty easy and cheap fix. New sensor from eurocar parts cost me about £15. You need to jack the car up, left front wheel off. The switch usually has a green plug connector, squeeze to pull off. The only thing to be wary of is unless you bung the hole quickly you'll drain your gearbox oil. I did an oil change whilst I was at it. The oil was £35 off eBay. Took about an hour in total with the oil change.