Its a shot in the dark but does your control work on all speed levels? The reason I ask is because on my wife's truck, it was doing the same thing.I replaced the blower motor and problem solved with the weak air flow from the vents.
Add Year, Engine, Tranny to Signature for better & accurate replies in future;
Common issue is;
Leaf & Debris Build-Up Location pic in 80-96 Blower Motors & Ducts
Source: by Steve83 (Steve, That dirty old truck) at SuperMotors.net
This shows where leaves build up in '80-96 truck ductwork. If they build up around the resistors (green), the entire truck can catch fire if the motor (yellow) is NOT at full speed all the time. The red pointer is where the cowl drain boot should be.
This is where leaves build up, on both sides of the truck. They come thru the wiper valance slots ('80-86) and between the windshiled & its lower trim (all). This valve is more easily accessed by removing the 5.5mm (7/32") screws from the edge of the wheelwell
Insufficient Heat TSB 88-09-10 for 87-88 Bronco & F Series; "...Insufficient heat inside of vehicles equipped with or without air conditioning may be caused by the heater air baffle missing from the plenum chamber. The heater air baffle (temperature Blend Door) prevents cold air from leaking in around the heater core. To correct this, check to see if the heater air baffle is present. If the heater air baffle is not present, install a heater air baffle using the following service procedure. SERVICE PROCEDURE: 1. Remove the heater core access cover to see if the heater air baffle is present. If missing proceed to Step 2. NOTE: IF THE TEMPERATURE BLEND DOOR SHAFT CAN BE SEEN NEXT TO THE HEATER CORE, A HEATER AIR BAFFLE NEEDS TO BE INSTALLED. 2. Remove the heater core. 3. Remove the heater core face gasket. 4. Install the heater air baffle, (E7TZ-18D416-A) by inserting the pin on the bottom of the baffle into the lower surface of the plenum chamber. 5. Reinstall the heater core face gasket. 6. Reinstall the heater core. 7. Apply a bead of Ford Silicone Sealer, (D6AZ-19562-AA) around the heater core access cover. 8. Reinstall the heater core access cover. PART NUMBER & PART NAME: E7TZ-18D416-A Heater Air Baffle D6AZ-19562-AA Ford Clear Silicone Sealer. SUPERSEDES: 88-04-14; OPERATION DESCRIPTION TIME: 880910A Inspect only 0.3 Hr.; 880910B Inspect and install heater air baffle 0.6 Hr
more heater info LINKs in my site @ttp://www.broncolinks.com/index.php?index=86
Lack of Heat, Temperature Blend Door Cam Spring Does Not Blend Door to Seat Properly TSB 92-4-13 for Vehicles Built Prior to 11/11/91, 92 Bronco & F Series & F-47
Low or No Heat/Air Conditioning, High Effort to Turn Temperature Control Knob, Poor Temperature Modulation TSB 96-13-7 for 92-95 Bronco, F Series, F-47
Low/No Heat When Temperature Knob Is Set At Full Heat TSB 94-20-16 for 92-94 Bronco & F-150-350 Series
Odors, Eliminate Air Conditioning Musty & Mildew Type TSB 98-2-7 for 92-96 Bronco; 92-97 Aerostar, Econoline, Explorer, F SUPER DUTY, F-150, F-250 HD, F-250 LD, F-350, Ranger & many others
Poor Heater/Defroster Performance in Engine Coolant Concerns TSB 93-24-08 for 84-94 Bronco, F Series, Aerostar, Econoline & many others; Blend door operation, Check blend door for binding and proper operation; Mode door operation. Check mode door for binding and proper operation. Blower fan operation, Check blower fan for proper operation and control. Heater core plugged, Flush cooling system. Coolant level, Check level in radiator and coolant recovery reservoir. Leakage, Check for leaks using pressure test. Thermostat operation, Check Thermostat. Refer to "Thermostat Diagnosis" procedure.
Check to see if the top radiator hose is hot after running the engine to normal temp a sign the T-stat is working properly, did you replace the heater hoses recently .....check the inline and return line are hooked up correctly.
I would suggest you "backflush" the cooling system and clean out the inside of the engine, radiator, heater core and wash any reusable radiator/bypass/heater hoses in detergent and that will certainly help with heat assuming controls, blend door and blower motor etc. are all functioning properly.
Also if the issue isnt fixed already, pull the heater control panel and look at the plug on the fan speed. Pull it off and inspect it. Mine did the same as the OP and my plug was melted. if yours is the same way hit up a junk yard and get a different one. Just make sure you cut the wires back so u have room to splice the wires to your harness.