You have installed a rectifier and depending on your volt meter you may only read 0.707 of the peak voltage. Your actual voltage may be 1.414 x 12= 16.9V.
You can install a regulator
https://www.motosport.com/product/?c...445339&variant[TRH0001]=TRH0001-X001-Y001
or a LED bulb that has a wider range of input voltage.
https://www.ebay.com/itm/H6-BA20D-DC...0b90bd07b5a0ba