Forms :: Selecting And Passing Values For A Query?

I have a form in which the user selects values above from a LOV and then selects a run query button which then uses the values selected in the LOV to refine the query and output the appropriate values.

What i have done in the pre-query trigger is: (the block i am querying is called cars where the block i am selecting values from the LOV is called selection)

:cars.make := :selection.make; (repeated for other fields)

this worked fine untill i introduced a from and to date in my form in which i passed the data through to the query like this:

declare
v_dates_between varchar2(1000);
begin
v_dates_between := 'reg_date between ''' || :selection.from_date || ''' and ''' || :selection.to_date;
Set_Block_Property( 'cars', DEFAULT_WHERE, v_dates_between ) ;

Now i have added the above bit of code the from and to date work fine, but when a car make is selected it still bring back every car make and not just the one selected.

is it possibly because i am setting the where clause in the block property? perhaps i need to now incoperate my other selections into this where clause? are the two blocks exclusive to eachother? and if i put the whole code into the block property I assume i will get problems. E.G. the user leaves a selection criteria blank (if they want to query all car models they wouldnt select one from the list) i assume the query would only return back values which contain no car model which would be 0 records.