This may not translate to the 99+, but I replaced the mirror on my moms 98 Suburban, she had power but that was it. The new mirror had defrost also. Just plugged it in and the defrost works without any alterations. I think that GM put everything there for every model, just took things away like the defrost/power and all that. Same with the trucks already being wired for a trans temp guage on the cluster just not being used.