The climate controls on your Volvo are all electronic at the control end, but likely have little vacuum motors to control the opening and closing of heater valves and air directional flaps in the innards of the dashboard. With the "mature" nature of your vehicle, it is sometimes a cracked vacuum hose or that the rubber end is getting a little hard and now leaking at the nipple where it is plugging into the vacuum valve. IF this is the issue, you MAY be able to here a hissing noise immediately after turning off the engine. Crawl down and get your head (ears) right under the dash. There may be a cardboard/plastic cover plate on the underside of the dash that you have to remove to get access.
But from the symptoms that you describe in your post, "Temperature dial has lost it's clicks"... I SUSPECT that the contacts on the control module may be totally worn out for both the direction of flow and the temperature control. Modules are available on eBay/Amazon (usually used) and it should be quite straight forward on how to remove / replace. The link below provides both links to purchase the control and also some YouTubes on how to replace it.
Hey... do you have a Pick-n-Pull or similar self-serve auto-wrecker near by? Often you can see what vehicles they have on their lot. It appears that s70 v70 and c 70 Volvos from 1998-2000 all have the same control unit. I've also included the part number in one of the photos... do a search for that and you may find more hits.
I hope this helps you Joan!
1998 Volvo S70 2 0 Google Search