Mazda b4000 1994 mazda b4000has steam coming out of the vents

What is your problem?

Get the fill container replaced as soon as possible!! It's an easy job to do yourself.
Drain the coolant, replace the container, refill the system. Run the engine until at working temperature. Turn it off and let it cool down then re-check the coolant level.
If you are running the vehicle with not enough coolant you are basically 'cooking' the engine!
Running an engine 'Too hot' can cause parts to heat to the point where they will weld together causing Expensive damage.
Coolant runs at around 100*C so some steam is expected. If there is a lot of steam get the vehicle to a Good mechanic.

have an accredited ac specialist shop check and clean the gunk from the evaporator fins that may be blocking the air flow

Turn your recirculation off,air coming from outside.

There is a recall on 2012 mazda 6 , Here is what I would check out .
Recall Date:
MAR 11, 2014
Model Affected:
2012 Mazda MAZDA6
Summary:
Mazda North America Operations (Mazda) is recalling certain model year 2010-2012 Mazda6 vehicles manufactured September 14, 2009, through May 2, 2011, and equipped with 2.5L engine. In the affected vehicles, spiders may weave a web in the evaporative canister vent hose, blocking it and causing the fuel tank to have an excessive amount of negative pressure.
Consequences:
Negative pressure could cause the fuel tank to crack resulting in a fuel leak, increasing the risk of a fire.
Remedy:
Mazda will notify owners, and dealers will reprogram the powertrain control module and inspect and clean the canister vent line. If a web was found in the line, the fuel tank and check valve on canister vent line will be replaced. These repairs will be done free of charge. The recall began on April 30, 2014. Owners may contact Mazda at 1-800-222-5500. Mazda's recall number associated with this campaign is 7214C.
Potential Units Affected:
42000
Notes:
Mazda North American Operations


Read more: http://www.motortrend.com/cars/2012/mazda/mazda6/recalls/#ixzz3A3gO2Ac1

When I turn on the heat in my 2004 mazda mpv the front air vents blow heat but the rear air vents are blowing cold. I have adjusted every setting is there something I am overlooking.

where ever u see the steam is ur leak-- steam is the coolant burning off usually wont show a puddle unless hose leak,or pump leak-- if the steam coming from tailpipe possible head gasket went

don't insert nozzle in fill hole so deep. It is not getting the air it needs to vent out while your fillinh the tank. Also start off slow on filling the tank, so that the air will start coming out while gas is going into your gas tank. like is said, the tank isnot venting fast enough, to let the gas come into the tank.

Could be a blow headgasket. Is there any white smoke/steam coming from exhaust pipe?

if it is a cool mist ,it is the air conditioner freezing up ,check freon to see if it is low,,and low presure cut out switch to see if it is working corectley,,u might have a closed up expansion valve,,,,,if it is not a cool mist and smells sweet,,,it is the heater core,,,stop leak,,or replace heater core