How do I sign my wifes name on plnk slip of car. She died 12 years ago?

Check with your lawyer but you can sign your name then write "for" then sign her name and attach a copy of the death certificate.

To click on the link remove the space after the //

https://
thelawdictionary.org/article/how-to-transfer-the-car-title-of-a-deceased-person/

"Transfer of a Vehicle without a Probate

Unfortunately, the process of transferring the title of a vehicle without a probate varies pretty greatly depending on what state you live in. Most states require the deceased individual's property not to exceed a certain amount and it to be a certain amount of time since the person has passed away before the transfer of the title can be initiated.

Required Documents and Fees
While the documents needed does vary slightly from one state to the next you need to have all of the following in order to transfer the title in most states: certificate of the title, affidavit to transfer without probate, odometer disclosure statement, death certificate, and transfer fee."