«to was»?

Good day/night to all!

«I had seen the car for the first time with the westering sun shining fully on the flawed windscreen, and I had seen it the second time in the shadows of LeBay's garage. Now I was seeing it under these high-set fluorescent tubes. Three different kinds of light, and all it added up to _was_ an optical illusion.»

Shouldn't the ending have been: «...and all it added up to _be_ [or «to produce», but not «to was»] an optical illusion.»?

I just don't get the structure of the original phrase.

No, this isn't an infinitive.

This is how you need to splice that part:

/all it added up to / was an optical illusion

It's the phrasal verb:
to add up to something
which I think you know:
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add up to sth

to become a particular amount
The various building programmes add up to several thousand new homes.

to have a particular result or effect
Trains are frequently cancelled and always late, all of which adds up to a lot of frustration for the passenger.
Whether such proposals add up to any real help for the poor remains to be seen.

(from Cambridge International Dictionary of Phrasal Verbs)
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