Without spoiling the whole thing (there are maps on the Internet indicating directly where to find each upgrade), especially in Alaska it helps to travel down EVERY road on the four maps, and look around.
When unlocking all watchtowers, there are still 40 percent of all upgrades missing on the maps, which have to be found simply by exploration.
Except for one to two upgrades, all hidden ones can be found „off-road“. Some however require you to travel down steep slopes and look for them on the coast lines.
Hint: one (the increased suspension for the Hummer H2) is located to the very south of the North Port map, near the area where a big rig must be transported over a nearly inaccessible flooded road. The upgrade is located near some rocks on the southernmost shoreline.
In addition, it helps to switch to nighttime via the map (by pressing Y repeatedly). The dashed yellow line that surrounds the upgrades is nearly invisible during the day due to the amount of white snow, but at night, the dashed line is greatly visible from afar.
There are 23 upgrades in total, 10 of them are hidden.
Edited:
Because there are requests, you can also check out
https://www.maprunner.info/ for detailed information and analysis of each map, mission or upgrade.
Personally I think it takes the fun away, because simply looking and going places is equally rewarding.
I take no credits for the maps.
Edit no. 2:
As of version 6.0 and the new DLC from the season pass, you can unlock a scout radar trailer in the new Kola peninsula region on the map of Lake Kovd.
Once the main contract "Forgotten Prototype" from the Kola Expedition company is finished, you must have returned a small scout trailer to the small town near the starting garage.
You then own the trailer , which can be bought from any trailer storage in the game.
The trailer van requires fuel for every scan and unveils undiscovered parts of the map in a 350 m radius around the trailer, uncovering every mission, building and upgrade around it.
That's also a possibility for finding hidden upgrades in Alaska.