s13 harness installation
 

I'm thinking about installing a Sparco racing seat with 4 points harness in my s13 for autocross, however I want to keep my stock seatbelt for daily driving. Is there any way I can have both seatbelts at the same time? Has anyone ever done this? Thanks.

Get a harness bar, retain stock seat belts.

yeah you dont have to do anything special but instal it and keep your oem seatbelt!
you never see it cause most people end up finding it uncomfortable and deleting it.
both will fit no problem just install the seat and harness.