Also thanks for the ones explaining how to reuse the connectors of exhausted 9V batteries.
Thanks for your time!
Step 1: Elements:
1 2x4 plain brick of LEGO
1 2x4 "normal" brick of LEGO
3 5mm 3V LEDs
1 Connector of an exhausted 9V Battery
You may (will) need a solder an lead.
Of course you need also a working 9V Battery...
Step 2: Drilling the Holes.
Remember that theLED s must be 5mm dia, exactly the inner diameter of the factory brick's holes.
Step 3: Trimming the inside.
Step 4: Soldering.
You must connect it anode to catode; if you don't know, simply try and try until they light..
I didn't use resistors because 3V (white) LEDs can work even at 4,5V so there is no fear.
One more thing: You must insulate contacs, with tape or paper as I did, leaving the main contacts out.