Sorry to dredge up an old post, but I'm working on the same thing. I've got my tape deck out, and using my DMM I did find that when there is no tape in, the circuit between the two outermost pins (white and red supposedly) is open. However, if I put a tape in (or trigger the mechanism to simulate a tape in), the circuit closes. I do see a few ohms of resistance, but it could also be my cheapo Harbor Freight DMM talking.
Either way, I think it's safe to say that this is our tape insertion switch. My only remaining question is whether blue is the ground for black left and yellow right, or whether it uses chassis ground. Guess I'll find out!