If you don't have the power lumbar support, try this from Corbeau, a seat mfg.:
"Adjustable Lumbar Supports - List Price $49.00 (US)*
*Please contact an authorized dealer for specific pricing. List price (MSRP) is for general reference & does not indicate actual selling price. "
http://www.corbeau.com/brochure/lumbar.html ---
or this one made for the Miata by Lewis..(will it fit, i think so)
http://www.miata.net/lewis/ $49.95.
"This unit installs in minutes with NO tools needed. Just slip it behind the lower back cushion and run the air hose along the center console out of sight. Adjust air to the perfect comfort..."
http://www.miata.net/lewis/images/lumbar.jpg