Really. Doggy sweat in carpets lasts forever. I nicked my mums Hoover constellation - it was too heavy for her - in about 1973, and had it for at least 5 years after that. It never stopped smelling of dog flavoured fluff that presumably was stick on the filters.
Unless you strip out all the carpets , upholstery and underfelt and wash or replace them, I doubt anything will help.
Five years ago, at the end of March, I bought my present car (Focus hatchback). It had a pleasant, 'just valeted' smell, with maybe just the slightest suggestion of cigarette smoke. However, as the weather warmed up, it absolutely began to reek. The dealer where I got it gave it another (free) valetting, but again, after a few weeks, it began to smell of cigarettes.
I myself tried several of the usual deodorizing treatments, but with little effect. I finally traced the worst of the smell to the 'fuzzy felt' rear shelf (boot cover), so I took it out, laid it on the lawn, soaked it thoroughly with a hosepipe, then gave it another deodorizing (using hot water). Finally, I rinsed it using the hose. When it had dried out, it did smell infinitely better.
For a few days the car didn't smell too bad at all. However, after a few days, again the cigarette smell began to assert itself. So, again, shelf out, and treatment repeated.
As the summer progressed, I think I did the shelf four times altogether. However, it was probably only at the onset of the cooler weather when I realised that the smell was beginning to abate.
But, to cut an increasingly long story short, it took around four years for me to become more-or-less unaware of the smell. I think the moral of the story is simply not to buy a car which smells!
Two things, if there's the slightest dampness, this won't help. I don't drive much so in the winter I leave one of these "home dehumidifier tablet" jobs in the car to soak up any dampness after journeys and that makes quite a difference.
Also, you might want to try borrowing a steam cleaner with small attachments - this made the world of difference to our house carpets when we moved in. Any dog hair curls up and can be hoovered out and the steam seems to nuke the smell in a way that even the hottest water doesn't.
After that you're onto "carpet deodorizers" and the "industrial" type air freshers (can't remember the name but they're almost spherical white pots in "blue" or "green" flavours).
Only bad experience I've had is a spilt milkshake that nobody told me about for a few days, by then it reeked. I cleaned the original mess but the smell was still pumping out of the vents. I sprayed a fair amount of Febreze all over the interior and a tiny bit down the air vents, kept the aircon/heat fans on high and the "recirculate" option switched on for a few days' worth of trips, and it got rid of it. The OP might have some hairs/odour stuck inside the ventilation system, if that's the case then using the heaters will definitely "bring out the aroma" of wet smelly dogs!
I always keep a bottle of Febrezee in the car and occasionally spray a little bit on the seats, boot, carpets, head lining. You never get that "new car" smell back but it's the next best thing I reckon.
Funny that, I too bought a used Focus that stunk of stale smoke. Two treatments of AutoGlym Interior shampoo managed to clear it + a bit of time. All interior surfaces needed treating though.
This car is surprisingly dry inside. Unlike the other one which is the same make, and year of manufacture, has a habit of steaming up in the evening if the sun has been out during the day.
I do have access to a commercial steam cleaner, and I used to service them too. I used one to clean a 'gold effect' shower door and ruined it. Ooops.
We have cats (wish we didn't) and one likes to leave 'messages' in places. I was told about BoS and have tried that with limited success. I found that Surgical Spirit worked better for that, but SWMBO does not like the smell of SS, otherwise I might have tried that.
The smell is a bit 'greasy', not at all pleasant, nothing like the Alsations and Poodles we used to keep.
Whilst out shopping, saw 1001 Car Upholstery and Carpet Cleaner spray and will try that next.
Been wondering about making a homebrew Ozone genny for a while, as I have breathing difficulties - probably a reaction to the cats, maybe / maybe not, we seem to get a lot of mould in the bedroom. SWMBO used to be in microbiological development, and I'm sure that she used to bring home the fungi, bacterium, yeast, spores in her clothes and hair. Unavoidable really.
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