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Don't you somewhat have this feature already if you simply use keywords instead of the dropdown? Similar to how it works if I select all F-250 all models? I agree having multi select on make and model would be great, easier for the casual user, and potentially more reliable depending on how you implement it but there is a way to do this in the mean time. .
For example if I am searching for a heavy duty truck, want to look at Ford, GMC and Chevy I just use the below in the Keywords section.
Ford|Chevrolet|GMC F350|F-350|"F 350"|F250|F-250|"F 250"|"2500"|"3500"
This tells the search it must contain one of the makes in the first set of brackets and one of the models in the second set of brackets. If I want an additional feature like package levels with leather trim id just do the below, and then it has to also match a 3rd criteria.
Ford|Chevrolet|GMC F350|F-350|"F 350"|F250|F-250|"F 250"|2500|3500 Lariat|SLT|LTZ
Not sure how complex you can get before this breaks things, but basically if you want an "and" match conditions, put a space between the words, if you want an group of "or" match conditions, use a "|" between words, not to hard to figure out if you play with it a bit.
On the reliability side, the one wish I'd have is if you implement multi select is please don't simply just convert it to keyword like above, while it works and is the best we got today its problematic and can result in a lot of false matches since it doesn't use the make and model object in the post and does a search text. Would love if you could find a way to limit it to the actual model and make fields so these garbage advertisers aren't matched.
Think that would be easy enough with craiglist and can be fixed with your current F-250 all issues if you just change from converting it from query search to auto_make_model, then the garbage doesn't match.
AKA: today it sets the url query to: query=Ford+%28"F-250"%7C"F+250"%7CF250%29
If you change that to set to: auto_make_model=Ford+%28"F-250"%7C"F+250"%7CF250%29
Then craigslist would stop matching me to a 1976 ford pinto when I searched for an f250 cause some a**hat likes to put false key words in the body of the post.