Thanks very much for the reply Lois.

So you're saying the keyword=whatever will always override and negate any greater than / less than options specified in the search form and there's no way to change that?

I guess I can add text fields to my search form such as:
manufacturer [ ]
model [ ]
year newer than [select]

I was hoping I could use:
keyword [ ]
year newer than [select]

where the keyword would search within 'model' 'manufacturer' and 'description' at the same time and still be limited by the greater than year selection, but I guess that is impossible as keyword will always negate the gt- search on the year field?