Will 2003 silverado transmission fit in 1999 sierra?

Yes, a 2003 Silverado transmission can fit in a 1999 Sierra, as both vehicles share similar drivetrains and components since they are part of the same GM platform. However, you should verify compatibility regarding transmission type (automatic or manual), engine size, and any necessary adapter or harness modifications. It's also essential to check for differences in the transmission control systems and ensure that they can be integrated properly.