limited heat??

2004? I'd have to research it, but it may have a vacuum-operated Control Head. Check for small vacuum lines going to-and-through the firewall and if they exist, make sure they're all connected and not leaking. Does it have an electronic temperature readout or anything? Another possibility is the Actuators, which are just small motors which move flaps under the dash that re-route the air. It could have one or bad actuators... At the shop I work at, we have a Chevy 1500 from the early 2000's and it will randomly cycle itself from A/C to Full Heat and to reset it, you've got to cut the ignition off in Neutral or Park, then restart the engine, which automatically resets the Actuator(s). You may have a similar issue. Actuators aren't all that expensive but are almost always very labor intensive and I don't recommend taking on that job unless you're very mechanically inclined and competent, because often, you have to pull the entire dash out. The last Blower Actuator job I performed as a Tech before I got into sales took me 9 hours (being very careful) and paid 10.5 hrs, which is over $1,000 in labor alone at an average rate of $95/hr here in Florida. Good luck.