None of the flavors were overpowering (it didn't taste mostly like onions or jalapenos or garlic)
That statement provides the key you can work with:
prepare a bowl of each ingredient.
make a reasonable guess as to the proportions and mix a sample from a small amount of each.
taste the sample:
if one ingredient is very overpowering, discard the sample and ...
First, ask if they cook any of the ingredients.
It's likely that they are all raw.
Next, I would probably start with a mix with an approximate base that has the same color as the original sauce ( e.g. 1:1:1 with one clove of garlic).
Save some of it on the side to use again, then add some of each ingredient to your base. 2:1:1:1 / 1:2:1:1 / 1:1:2:1 etc....