I found a diagram of the fuse panel for a 2004 Ion and it appears the relay you are referring to is Relay 28. I see there is also a Run/Crank fuse (30A) in location 38. Have you confirmed this fuse is making a reliable connection?
Many years ago my now wife had a 1995 Saturn SL that randomly would stall when the rear window defroster was turned on. The car was still under warranty and the dealership claimed they fuse connections were not making good connection with the fuses and the added current draw of the rear defroster would cause the intermittent connection and stalling. They replaced the entire fuse panel under warranty and it did seem to resolve the issue. I'd like to think that if this was a common issue in 1995 (it didn't take the dealership long to determine this was the issue, leading me to suspect it was a common issue) they would have made improvements by 2004, but it's possible they didn't.
-Rod