Assuming this is a floor shift car the linkage from the trans to the steering column may need adjusting. If you have to adjust the switch it's located on the steering column near the base on top.
Thanks Guys,
The link supplied by Thecatman was right on. I loosened the bolt on the cross rod underneath the car and moved the linkage until reverse light came on, then re-tighten bolt, very easy.
With the help of my 11 year old son it was done in 5 minutes. Thanks again