I have a customer's 99 S80 that I decided to replace the heater core myself. This is not a terribly difficult job, but there are a lot of steps. There seems to be a bit of confusion on the process. It is NOT as easy as an 850 or S/V70. Here are the general steps.

1.) Disconnect battery and remove from vehicle.
2.) Remove under-panel sound barrier along with driver's console side.
3.) Remove driver's airbag, turn signal lever module/clock spring assy.
4.) Remove shiftlock cable from ignition tumbler.
5.) Remove wiring guide from bottom of steering column.
6.) Remove steering column assy.
7.) Remove driver's carpet (pretty easy).
8.) Remove brake light switch assy.
9.) Disconnect heater hoses at firewall (replace them with new when finishing).
10.) Remove clips for pipes at heater core.
11.) Remove torx screws holding heater core in place.
12.) Slide out heater core.

You should put the system in full heat position prior to removing the battery so that the air door arms move out of position for heater core removal.

You're probably going to have wet carpet anyway requiring a cleaning so I removed it before dumping another pint of coolant on the floor.

Installation is reverse of disassembly.

Before installing carpet, sound panels and console side, check for leakage.

The biggest pain is the clips holding the heater pipes in the core.

 

I see no need to remove the hoses in the engine bay. The hard lines will move without removal and you'll save a ton of time trying to wrestle them off. There's no easy way so take my advice on this one.

I can't say enough how much easier it it to do this job without the steering column in. You will have to lower it anyway so take the extra steps of disconnecting the wiring and the bolt at the steering shaft coupler. To remove the shift lock cable you'll need the key in the ignition and turned to the 2 position. Also, be sure to relock the steering wheel before you pull the column to prevent the wheel from turning because that would be disastrous.

With the column out you will have a ton more space to access the hard lines and those pesky horseshoe clips. The entire removal and replacement will be easier and you won't scrape the sides of the new core, possibly damaging it. It slides in and out so easy it's almost scary.

I also removed the driver's seat because I'm 6 foot 3 inches tall and weigh a measly 275 lbs. While this was nice during the installation it was a hindrance when I was reinstalling the steering column. I did get to clean really well so it's not all bad.

Oh yeah, I just disconnected the battery instead of removing it. Again, I seen no reason to remove it.

 

Robert has a good video of how to do it. I did this a year ago and no problems at all even tho I did bend some of the fins (just like in the video) No leaks, working great.