I'm not quite sure exactly what you want to do, but assuming you simply want an LED to light when a pack is plugged into that particular plug, but not otherwise, there's a few ways to work it.
Right now I'm assuming that your packs are going to have only a main + and - connection coming from them to your bike. If they have balance leads, too, then there is a slightly different and better way, posted after the first one.
For one of the simplest methods, it's going to "waste" a little voltage coming from the battery pack, though. There are "better" ways to do it, but they get more complex. If there are balance leads it's almost identical, but doesn't waste the voltage or require high-current diodes.
For this one, you need the following parts, for each pack connection:
Diode capable of handling reverse voltage at least as high as the pack being plugged in, and capable of handling the full maximum peak current that you would ever draw thru that pack. If the current is low enough, you can use the 1N400x series. If current requirements are high enough, you may have to use heatsinked diodes.
Low-wattage resistor (1/4w should be fine, 1/2W or higher will work but will be larger), calculated to limit current thru the LED to below the maximum allowed by that LED. Usually 20mA or less, but it depends on the specific LED and what is required by it to emit the amount of light you want from it, without burning it out.
LED of whatever color and brightness you're after.
Interconnect wire, can be pretty thin (24g if you want really thin wires, would be fine--there's going to be very little current flowing here).
All of the connections are on the bike side of the battery harness, not on the batteries themselves.
Cathode of Diode gets spliced into the controller side of the main + wire of a pack's input plug, on the bike side (not on the battery). Anode goes to the pack side of that splice.
One end of the Resistor (either end, doesn't matter) gets connected to the anode of the diode, and the other end anode of the LED.
LED's anode gets connected to the resistor per above, and cathode goes to main - wire of the pack's input plug.
Thus, the diode blocks any thing coming back to the LED from any other packs that are plugged in, but when you plug a pack into that connection, it does provide power to the LED, which lights up.
Alternately, if you have balance plugs on the packs/bike, you can use MUCH smaller diodes--the 1n400x series definitely works. Everything else is wired the same, except for the cathode and anode of the diode. That then gets spliced in series of the *balance* plug on the bike side of things, cathode on the bike side, anode on the pack side, on it's lowest + cell wire, instead of the main power wire. That means there's no high current ever flowing thru it so it doesn't need a big diode. Just keep in mind that if you have anything that monitors the cell voltages thru those balance plugs instead of the main leads, it will read voltage wrong on that cell by the amount of drop across the diode. And you can't recharge thru that particular balance wire either, cuz the diode will block it--but normally you probably don't do that on the bike itself anyway.
Resistor stil goes to anode of the diode, and LED still goes to - wire of the pack (but it can go to the balance lead instead of the main lead.
This latter method is better than the former, but only works if you have access to an individual cell voltage, via balance leads/etc.
Slightly more complicatedly, you could use connectors for the packs that have extra pins that are only connected to voltage if the packs are plugged in, with the resistor and LED in series with those pins.
