Understood. I only meant to suggest that the ramps signal, when it's working, can sink/supply a lot of current on it's own. Per the other suggestions, I would do things in the following order:
1. Closely inspect the heat pad, to ensure nothing has damaged it in such a fashion as to alter the current path. Unlikely, but what the heck.
2. Replace the relay. It's the cheapest/easiest thing to try.
3. Replace, or debug/repair the RAMPS. This is costly in either time or money, but is the next most likely problem area.
4. Fiddle with the firmware. This seems like the least likely point of failure and you might spend a lot of time trying to find the problem.