I know I am replying late to this thread, but I too have a front mount sensor that i want to mount to the rear wheel so that i can get a speed readout while on my wind trainer. I followed the advice above, but to no avail...splicing the wires did not work. After a few hours a swearing and cursing I went to the local Cyclepath shop for help. Apparently, for some cyclecomputers, there is an invisible coating on the copper wires that must be burned off prior to splicing. If you peel away the protective plastic casing, then hit the wires with a cirgarette lighter you will be able to splice in an extension wire no problemo. They also said that old headphone wire works the best. Hope this helps you and saves you some aggrivation. I wish this solution had been posted somewhere before I wasted a few hours "discovering" it the hard way. To think, I almost went out and spent $100 on a new rear mount speedo...
Cheers.