I have an 1989 Cad sedan deville, that will not blow either heat or ac into car, I was told that the Plenum had to be replaced, I have replaced the blower motor and fuses. I do recall that a mechanic knocked on something under the hood (near firewall) and was able to get heat to blow back into car, now neither will blow into car ( heat or ac ).

Also, I would like recommendations on swapping out airbag suspension with that of something like Monroe kit. thanks