Wondering if anyone has ran into this problem. 69 Z28, having an issue with the turnsignal fuse blowing. Only happens if the radio fuse is in (the one above the turnsignal fuse). With the radio fuse out, the turn signals operate normally, the 4 way flashers are on the fast side. I plan to replace the flasher and see if that fixes that issue (the fast 4 way flash). The one that has me perplexed is why does the fuse blow only when the other fuse from a totally seperate system is in.

Additional info: The dash wire harness, the fuse box, and the engine bay wire harness are all repro, the intermediary and trunk wire harnesses are original. thanks in advance