Hi, and welcome to Just Answer!
If you have a relay, might as well put 'er in.. You should be able to find it in the fuse box under the hood (should be a chart on the inside of the lid).
If that doesn't take care of your issue, we are going to have to do a quick test at the blower motor when the problem occurs...
When the blower is NOT working, try this.. find the two wires that plug into the motor, and check each one for 12V with a test light (still plugged in, fan on HI)
if you have 12V on the green wire and nothing on the black, you need a blower motor..
if you have 12V on both the green and the black, you need resistor (most likely).
if you don't have 12V on either wire, give a shout back!
The important things to remeber are, the fan must be broken (not working) plugged in, and set on high wtih the key on..