Certified car seat technician here, your convertible must be anchored by the top tether if you are using a 5 point harness. If it is a convertible highback booster, and the 5 point harness is not being used, then you disengage the top anchor.
Your vehicle's instruction manual will point out the locations of your anchors and it has been law that vehicle's must have them coming from the manufacturer for quite sometime.
Convertible vehicles pose a problem for anchors as the room immediately behind the vehicle's seat most likely moves to accommodate the drop roof. Thus you cannot install a convertible car seat in a convertible car properly.
Let me know if you have any other questions.
1968 Impala: Status: Stored
1977 Dodge Triple E RV: Sold
1989 Mercedes Benz 420 SEL: Sold
2008 Mercedes Benz C230: Cruising
2000 Bluebird TC2000: Build phase of skoolie project
2018 Rav4 XLE: New baby friendly daily