I have no power to the heater blower switch

Gauge fuse 10A in left kick panel power the control coil of the blower relay
Alt 80A fuse in the underhood fuse box powers the contacts of the "heater" relay
Once the heater relay contacts close, power flows to the "heater fuse" 40A in the underhood fuse box.
From that fuse, power goes directly to the the blower motor.

So check all three fuses AND the relay

Since no vehicle is specified, make sure you check the engine compartment fuse box(es) for A/C fuses AND the under the dash fuses. Some makes have more than one fuse for larger capacity components.

Most blowers use a high amount of power, and there is normally a relay (connecting high powered items to a low power switch)

If you are qualified, you can hook up a separate switch to the battery to check your blower. Removing any A/C / heater core type component normally requires removing the dash, and verifying the blower is the problem before removing it could save a lot of hassle

There are two fuses for the blower. One for the relay coil and the other for the relay power. One is a 5a in the interior fuse box. The other is a 40a in the engine compartment fuse box.

Smack it and see if it comes on. If it does then the motor brushes are bad. If it does not come on, check the fuses. If the fuses are ok, check the voltage on the plug to the motor.

2 fuses to check, Kevin: in the instrument panel fuse block, 20 amp fuse #12, Htr-A/C. In the under hood fuse block, Frt HVAC fuse, 30 amp.

The new resistor you put in, was it called a "Blower Motor Resistor and Relay Pack" ? It appears there is a relay there with the resistor, but appears to only be used with htr on high speed. However the low speed wiring is also routed through this relay. If it was the Pack you put in, guess that is not an issue.
In the Heater and A/C Control Module is the blower switch. The power in to the control module is a brown wire (from the #12 fuse in instr. panel). With key on should be hot. It appears the brown wire goes to the mode switch and then to the blower switch. Check for power in, and check continuity of both switches. Or check if blower switch has power to it, then check for continuity in it-it should have continuity in all speeds.
If all is good so far, you need to check if blower motor ground is good.
Here can be found a wiring diagram for your Astro. Look for heater controls under the system called "HVAC". Every wiring diagram and TSB for the Astro (or any make, model) can be found here:
http://www.bbbind.com/free_tsb.html
Good luck.

REPLACE THERMOSTAT.IF ALREADY DONE.CHECK HEATER BLOWER MOTOR SEE IF POWER GOING TO HEATER BLOWER MOTOR.IF YES BLOWER MOTOR BAD.IF NO POWER GOING TO BLOWER MOTOR CHECK AC AND HEATER CONTROL FUSE AND CHECK BLOWER MOTOR FUSE AND RELAY IF ALL IS GOOD CHECK SEE IF POWER GOING TO AC AND HEATER CONTROL SWITCH IF YES THE AC AND HEATER CONTROL SWITCH COULD BE BAD.

CHECK BLOWER MOTOR RELAY ALSO, AND USE A TEST LIGHT TO SEE IF POWER GOING TO BLOWER MOTOR, IF YES BLOWER MOTOR FAULTY.IF NO POWER GOING TO BLOWER MOTOR AND FUSE AND RELAY OKAY CONTROL SWITCH COULD BE BAD,A RESISTOR BAD MOST OF THE TIME IF YOU DONT HAVE BUT A FEW FAN BLOWER CONTROL SPEEDS.

CHECK BLOWER MOTOR FUSE AND RELAY.SEE IF POWER GOING TO BLOWER MOTOR YOU HAVE FAULTY BLOWER MOTOR.IF BLOWER MOTOR FUSE AND RELAY OKAY.IF NO POWER GOING TO BLOWER MOTOR BUT POWER AT AC AND HEATER CONTROL SWITCH. BLOWER MOTOR HAS POOR GROUND OR AC AND HEATER CONTROL SWITCH IS BAD.

CHECK HEATER AND AC IGNITION 10 AMP FUSE UNDER HOOD JUNCTION BOX.IF ALL IS GOOD CHECK BLOWER MOTOR FUSE AND RELAY.IF ALL LOOKS GOOD. TURN ON CONTROL SWITCH TO CHECK TO SEE IF POWER GOING TO BLOWER MOTOR IF YES BLOWER MOTOR BAD.IF NO POWER GOING TO BLOWER MOTOR FUSE AND RELAY GOOD.YOU HAVE BROKE WIRE BETWEEN SWITCH AND BLOWER MOTOR OR CONTROL SWITCH FAULTY. TO GET TO BLOWER MOTOR ITS LOCATED UNDER THE DASH ON PASSENGER BEHIND GLOVE COMPARTMONT YOU DONT HAVE TO REMOVE GLOVE BOX ITS JUST BELOW IT FAR BACK.YOU NEED TURN OFF RADIO FIRST THEN DISCONNECT NEGATIVE BATTERY CABLE WAIT 20 MINUTES THIS DISABLE AIRBAGS AND THE BCM.REMOVE BLOWER MOTOR HOTWIRE IT TO SEE IF ITS WORKS. IF NO BLOWER MOTOR NO GOOD.IF BLOWER MOTOR GOOD THE AC AND HTR CONTROL SWITCH IS BAD DEALERSHIP FIX.

check switch at contacts for continuity. Check heater blower fuse and relay If fuse is blown and new fuse blows right away there is a short to ground or power. (either in cab or under hood in power distribution box). Check wiring running to blower for possible short. Check blower.