1991 Mustang GT with T-5 nitrous shifter knob, problem

I have a 1991 GT 5.0 with the t-5 tranny and I bought a hurst shifter knob from summit racing and they told me it was good for 15amps I hooked it up to the nitrous cube relay and it won't pull it in. I have voltage on the relay when I push the button, if I hold it it blows the fuse I am tied into, but if I touch the relay wire direct to the battery it pulls in the relay just fine! the wire that comes out of the knob is scary thin! if anyone has had problems with this switch I would appretiate the help.