Input Voltage For MTH Marker Light LED's

5-6 Volts.  Normally those are in series.  If you had a pair parallel wired, there would be a limiting resistor too.  Probably about 150 ohms.  G 

A 100 ohm resistor from the 5V supply will light them fine.  3V will not light the series LED's, those ceramic LED's don't start lighting until they reach close to 2 volts.

You still need a current limiting resistor.  If you put 3V on one of those red LED's with no current limiting, it will be dead in short order.  For series use, you have to connect the negative lead of one LED to the positive lead of the next one.  Then you connect the remaining positive and negative leads that are free to the power source.

The stock MTH ones are simply wired in series, I just cut the insulation off a set to make sure.  All of the dual ones that I have start to light around 3.5 volts.  At 3.0 volts, there's nothing.  There is no current limiting, so at 5V you'd need the resistor I mentioned.

MTH has quite a few different variations on there LEDs.  Early where parallel wired, later versions where series, some had current limiting resistors under the insulation, others did not.  All depends on what application it was originally used for.

All MTH CV boards I have come across are 6V output.  Some are more complex than others.  150ohms seems to be the standard resistor MTH used on current limiting.  The MTH LEDs for markers are pretty hardy.  G

They may be "hardy", but over-voltage with no current limiting will kill any LED in short order.

I thought I had seen parallel connections, but the one I actually cut open was series.  I think the parallel ones were in factory stuff, most of the extra LED assemblies I have are from upgrades where they were not needed.