How can I decrease the brightness of these BA-15 LED bulbs
You could wire a resistor in series with the base. The lamps are claimed to be 2W so they should draw around 170mA from the 12V bus. Try about 10-50 ohms 1W to get the brightness you want. 

Of course you will no longer be able to safely install an incandescent bulb, in fact doing that could cause the resistor to get dangerously hot. You could add a small resettable fuse (polyfuse) to prevent that situation.  

As a bonus, the lamps should last much longer, especially in a warm climate. They are typically driven to within a millimeter of their lives because that saves the manufacturer money.