Please don't throw in the towel just yet for yourself or others.
What was the reason given by the dealerships, and what car are you attempting to retrofit with top tether anchors? ETA: I think I found the answer in a prior thread you posted about your 2008 Suburban. That model isn't retrofittable with additional top tether anchors through GM's retrofit program because it already has some factory equipped top tether anchors, and GM doesn't provide parts for adding more top tether anchors in GM models that are already factory equipped in some seating positions. The OP's '99 Chevy Silverado is pre-factory equipped and eligible for the GM bulletin Wendy cited upthread through which GM dealerships get reimbursed by corporate for providing one top tether anchor retrofit free of charge to owners of any retrofittable 1989-2002 pre-factory equipped GM vehicle model. There have been reports of lack of parts available anymore for some older but previously retrofittable models thanks to parts scarcity/GM's bankruptcy filing several years ago, and a few GM models that were never retrofitttable at all (like the Corvette, or certain model year Camaros), but there have been success stories as well if one is persistent and gets a dealership that isn't completely clueless (a recurring theme, sadly, and not limited to GM dealerships
), or can be clued in by involving corporate to educate the dealership about the retrofit policy.
For lurkers, if the car isn't a GM brand like the original poster's Silverado, Chrysler/Dodge/Jeep and Ford have top tether anchor retrofit bulletins/policies for their respective retrofittable pre-factory equipped models to provide as many free top tether anchor retrofits as the specific retrofittable model can have added. Toyota has a voluntary contribution voucher program for its retrofittable Toyota and Lexus models through SafetyBeltSafe USA for lower cost retrofits@$5 per top tether anchor voucher. Other vehicle manufacturers have retrofittable models and parts info. listed in the LATCH manual, albeit without free programs.