2007 Yaris replacement voltage regulator and wiring harness don't fit.

Hi,

I'm trying to replace the voltage regulator (in the alternator) and the wiring harness, but when I ordered replacements, they aren't the same size as the originals and are a tight fit, so instead of clicking into place, they just stick to each other and have to be pried apart with a flathead screwdriver. I'm just wondering if anyone has any experience with this. I'm considering returning the parts and having my mechanic do it instead, but that means he'll replace the entire alternator.

Thanks for your help.