Blower works; thermostat OK; AC works; No blinking lights...No heat; have climate control unit w/ 2 zones and temperatures for each zone.

The ECC units have a blower switch that has variable speeds, ie. not 1,2,3,4. If this is the case, check the level of antifreeze. Make sure the vent selector and blower speed are in the auto position before you put the key in the ignition.

You could remove the kick panel next to the gas pedal and feel the coolant tubes going to the heater core and make sure they are hot (and not leaking).

Because you say the engine temp is OK, it may well be that the temp dials are broken. For both of them to break at the same time will be quite rare, but possible. If this is the case, a new ECC unit will be needed. Junk yards are full of them (1998-2000).

Or is it the simple center dash vent is open to outside air?
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My name is Klaus and I am a V♂lv♂holic

I checked the heat supply and return tubes by the accelerator pedal and both are hot. I replaced the heat/AC control unit, and still have no heat. Could there be a damper that is stuck or linkage? If so where and how do I access it? Someone told me it may be a programming problem and to take it to a dealer. Is the climate control unit interfaced with the ECU? I'm still cold but appreciate your help so far.

There is a damper to the left of the glovebox that tends to fail, leaving the vent in the defrost position. This is not fun to fix as the location is terrible.
What happens is the hollow plastic shaft cracks, so the vent motor turns but the vent does not. The 'fix' is to remove the vent motor (3 #15 torx screws), glue and wire the the plastic back together, let the glue cure and put it all back together again. This is not a 'dealer' fix. Because your blinking lights of death do not come on, the vent motor is working correctly.

Beware, the vent motor is very close to the firewall and the 3rd screw that holds the motor on is hard to access and blind.

I currently have the vent blocked in the open position until the weather warms up and I can work on my 854 again. In my case, I have 2 blocked vents.

There are a couple of write ups with pics, one on Volvospeed and the other on Matthews volvo site. Your 1998 has the same system as the 850s, except that the glove box has to come out before you can drop the passenger side kick panel.

Read this entire post, quite long but educational:
http://www.matthewsvolvosite.com/forums/viewtopic.php?f=1&t=9982&start=14

You do not need or want to remove the entire dash! Placing the vent selector on defrost is a good idea before pulling off the motor.
While you don't have to remove the kick panel, I find it helpful when a screw falls down and lands on the floor board!
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My name is Klaus and I am a V♂lv♂holic