G'day AJ
Rectifier (or blocking) diodes are specified in current and voltage ratings like 1A, 10A and 50, 400 and 1000V.
The little indicator warning lamp would be 4W max, so that's about 300mA current draw which is why I said a 0.5A (500mA) diode would be OK. But looking at what's available on line, it seems 0.5A is not a common rating, so go for 1A. Since you're only dealing with 12V you don't need to worry too much about the voltage rating, but you pay a bit extra for the higher voltage ratings and they may be a little bulkier.
Diodes are dirt cheap. They usually come in packs of 10 but they're only a few cents each. For example, here's a pack of 10 for $2.60 including postage on ebay.
https://www.ebay.com.au/itm/IN4004-Rectifier-Diode-400V-1-Amp-x-10Pcs/302855164854?hash=item46839313b6:g:vaYAAOSw3ZtaLaEv:rk:2
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The postage would account for $1 of that so the actual diodes are about 16 cents each.
If there's a Jaycar shop near you, they'll sell them one at a time, but will probably charge you a rip-off price of 25 cents each. It'll cost you more in petrol to drive there!
Blocking diodes are normally silicon diodes. If looking on line, you might come across names like Zener or Schottky diodes. Zener diodes are for voltage control applications and are no good for your purposes so avoid those. Shottky diodes will work fine but might cost a couple of cents more. Only difference between Shottky and normal diodes is that Shottky have a lower voltage drop when passing current. We're only talking a few millivolts difference, so don't get all OCD about reducing voltage drop.
The diodes are not much fatter than the wires in your bike's wiring harness, so I'd suggest you simply solder them in line with the wires to the dash lamp and cover them with electrician's tape or preferably heat shrink tube. The coloured band (usually white or silver) on the diode indicates the current direction and should face towards your dash lamp.
That's pretty much all I can tell you about diodes.
Cheers,
Cam
