Speedometer problem

[eyonata]

I have a 90 240sx, and I recently replaced an old stereo with a panasonic CD player. The old one (Jensen) keeps dying out on me and it couldn't retain memory.

So I did a ghetto wiring straight from the battery past the firewall straight into the radio for the constant +12V for memory, the rest is just matching wire colors. The radio works, and has memory now. My gauge cluster was working just fine, the back light turns on and everything moves as it should.

This is where the problem started, I realized that I had forgotten to plug in the aux cable (for radio) while in target's parking lot. So I took my radio out (while engine running idle w/ light turned on, gauge cluster back light was fine) and just plugged in that cable. Suddenly the gauge cluster back light turned off, but I could see that the tachometer is set on idle as usual and the speedometer is fine too. So I thought that it could be a blown fuse somewhere, but the next day I checked out ALL the fuses, and they're all fine.

So I need help~!! What do you think could be the problem? How do I fix it? and how do I get to it?