I have a 2002 Sierra with the SL package which is all I need except for the power windows. How hard are these to install aftermarket? Any suggestions welcome.

If it was me I would by the oe window motors and regulator and switch them over to oe power windows. I installed the Spal window kit on my 79 blazer and they worked great for about a year and then the motors crapped out. Just my 2 pennies. Good luck with the switch. Oh you can get the oe replacements on ebay pretty cheap I have seen them as low as $35 BRAND NEW.