I have a 2 ton air handler, that uses electric heat, when I put the t'stat in cooling at set the fan switch on auto it works fine, but when I put it in heat the blower does not run unless I put the fan switch in the "on" position, it worked fine last year, there is a new circuit board and fan relay in it, and it does the same thing, could it be a bad t'stat, it's an old mercury style?

It could very well be the tstat, if you have a multi meter you can check the terminals in the circuit board for 24 volts. Check between are and w. and check between are and g. a quick cheat I always use when I think the tstat is bad is using a jumper between are and g and if it starts properly then the tstat is the problem.