1998 Tahoe 4-Door 4X4 350 107500mi Heater Front Blower Relay

Lately I've been getting blower failures in my front heater (same as A/C). Typically when starting the vehicle the blower works, but when I shut it (and the engine) off and later start the vehicle and turn the fan back on the blower (fan) does not work no matter what position the fan switch is in. The A/C is working (detectably audible) and the vent controls are working (also audible). Occasionally the fan will just turn on by itself if the switch is not set to OFF. Also, leaving the switch on any ON setting has been giving me 85% success rate in getting it ON after turning the vehicle OFF and then ON again. This all sounds to me like a relay problem. I don't have any wiring diagrams and would like to know where I can get them. I've seen some web sites but I'd like your opinion on getting them, and also on the diagnosis. I've found some harness parts sites like at rockauto. Com that indicate the relays are actually embedded in the harness (!) I'm over 50, not a mechanic and this type of thing is new to me; are the relays really in the harnesses? Or am I seeing things? I'm determined to understand this (for once !). THANKS. BZ.