Your multimeter's lying to you ;) A negative voltage reading basically means you've got the leads the wrong way round when measuring DC. So, the voltage is coming in through the black and out through the red. On your original question, I'd check continuity with your MM diretly into the 3 pins on the TPS. At 0 throttle, you should have continuity from middle to top, from 25% to 75% you should get no continuity and from 75% up you should get middle->bottom. If there's any cross-talk between the pins I'd take the TPS off and crack it open and you should be able to see where the problem is easy enough.