If you're smelling antifreeze in the vents, bypass the heater core or replace it NOW!!
The heater core is directly above the ECU, and it's possible to fry the ECU if you let it leak long enough.
The fun part is replacing the heater core- you must remove the ENTIRE dash to get to it. The only thing left in my car when I was done was the steering column and shifter. everything else has to come out. total PITA!!
Here's a link to a writeup done by an owner over in the UK.. same stuff, only the steering wheel is on the wrong side of the car.
http://www.newshampark.org.uk/bignissansold/j30/articles/blownecufix.htm
as for you guys with cool/cold air, you have several issues possible.
1. low on coolant, or air trapped in the heater core. when bleeding the system, make sure your heater is set to full hot, and the front of the car is jacked up as high as you can safely lift it.
2. clogged heater core. this happens quite often- well, not often but it's common on old cars. Over years of use, these heater cores get lots of sludge and junk built up in them because there's not a lot of water flow through them, so they collect deposits and the efficiency goes to crap. In order to fix this, you need to pull the two hoses off the heater core at the firewall and shove your water hose on FULL BLAST into them... flush it backwards and forwards several times, until the water comes out clean. this can take 15-20 minutes sometimes just for the heater core itself.
replace the coolant hoses, refill the system and bleed it out, then see what happens.
3. full-cold valve stuck open/closed.. there's a vacuum actuated valve on the firewall on the incoming line of your heater core. When you set the temp slider at the dash to full cold, this valve is supposed to close and block water going through the heater core. Sometimes this thing fails and gets stuck closed or partially closed due to corrosion.. the easiest way to check it is to simply bypass it and see what happens. you can buy splices/fittings for the hose at any auto parts store for about $1, and just take the hoses off both ends of the valve and stick them together.. then go for a drive and see if you have more heat.
so try that stuff and let me know what happens.