The speaker panel is held on with common adhesive (gel in a tube I'd imagine). If I were to put it back on, I would use 3M emblem adhesive available at PepBoys, AutoZone, Advanced Auto Parts, etc. Less is definitely more if you us the 3M adhesive. You can get it for lesss than $5 at Wally World:
The post below demonstrates me using it to re-adhere a JM speaker grille cover on. It worked 100% with zero issues.