Is there any way to add a cabin filter to my 98 like in the 00-01 models?

 

It's easy. Just purchase an OEM filter from a '00-'01 ES and install the filter to your car like you would normally in a '00-'01 (filter is installed in a small compartment located just to the right of the gas pedal on the floor).

 

any screws to remove? any carpet?

 

You can purchase the part from Toyota parts or a 3rd party. I've got the toyota one (ebay) and one made by wix (rockauto.com). The toyota one seems to be better constructed.
Purolator has instructions on how to install it.
See this link...
http://ecat.arvinmeritor.com/smartlink/?partnum=C25450

It is pretty easy but there a few things you may want to note...

1. There might not be an air filter there now. Some owners reported that when they went to change it, there was not one there to begin with.
2. Since you may not have anthing to refer to, pay particular attention to the air flow of how the air is flowing through the filter.
3. It requires you to get right at the gas pedal, lots of kneeling. Don't pull a muscle.

 

it makes a big difference. my car didn't have one when i bought it but when I put it in ahhh so freash and clean!!!

 

actually help remove the smelly odors. How long it lasts? Not long. But at least they have it to begin with.