On the 98 C/K truck, start with looking at the under-hood fuse/relay box. If you have a relay location noted as "AUX FAN" and 30 amp fuse marked AUX FAN. It is the factory location controlling ground on & off for this relay, by the VCM/PCM cavity 9 (DK BLU). B+ to the AUX FAN relay coil comes from the A/C compressor fuse 10A, but aux fan relay switch is 30A fuse in dedicated spot, in the same electrical center. If you have an empty spot with no AUX FAN relay listed, a tuner can add it to make the factory program for a towing package (I would guess). As for fans running with A/C compressor, the description for the "AUX FAN" if equipped is;
Auxiliary cooling fan provides additional cooling for low speed vehicle operation, extended idle, stop and go conditions, and running the air conditioning system.
This is from GM eSI (Electronic service information). Now this is one fan, but if re-programmed to make operational on a vehicle without it, you can use the same ground signal from the VCM/PCM to operate two relays with 30 amp fused supply and they will come on with VCM command, using engine temp inputs built in without adding any sensors or switches.
Note; if the cavities are empty, you use your dual fan kits relays, by just adding one wire (Circuit #473) to the VCM to operate the fans relays. Depending on your skills, it can be done fairly easy with terminals & wire from a local bone yard for the VCM.
Look up;
www.LT1swap.com and email Brendan to ask if he can do that for you. $75 is programming charge.
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Just look at your fuse/relay center to see if the relay and fuse is listed...Then go from there. It's probably a factory option. Item #1 is AUX FAN relay, #2 Fuse/Relay center, #3 VCM.