i have a 1989 sedan deville and it needs a new heater core, but im not sure how to install it, ive been searching the net trying to find info, does anyone have some basic instructions on this?

One bolt at a time, then disconnect the lines (after your drain the radiator), be careful to not get any remaining fluid in the lines to leak in the car, then reverse the procedure to install. Too easy.....

Welcome to AF, there are just about no non oem manuals out there for Cadillacs that are of any help. A subscription to Alldata.com may be one answer, or try E bay for cd based manuals.