I think of it like...which one of these is correct...

...the customers was...
or
...the customers were...

It seems that the 'was' or 'were' would be decided based on the subject, so then only the second would be correct. If "was" was acceptable in this sentence, then sentences such as "we was going to the store," or "they was regulars," would be acceptable as well--and as far as I know, they're not. At least, I hope they're not because they sound really wrong to me.

I like stuff like this too.