Summary
Building numbering in the City of Yonkers deviates from the practices commonly used in the rest of New York State. Buildings have a "legal address" that may be different from the mailing address commonly used to give directions. Waze house numbers and Place addresses (especially Residential Point Places) must use the mailing address, not the legal address. Do not rely on the State or Westchester County GIS (or scripts that refer to them), because these display the legal address. Instead, use Streetview (where possible) or the City of Yonkers GIS map to verify the mailing address.
Mailing address vs. legal address
The City of Yonkers, unlike most other municipalities in New York State, maintains a system of "legal addresses" for properties within its borders. The legal address is sometimes the same as the property's mailing address, but it can be different. Sometimes, when a property is located near a street corner, the legal address may be on one street and the mailing address on the other.
The mailing address is the address used by residents and businesses to give directions to their location, and so Waze house numbers and place addresses should always use the mailing address, not the legal address, when the two are different.
Finding the mailing address
Both the New York State and Westchester County GIS data provide only the legal address for properties in Yonkers. DO NOT rely on these sources to set or change house numbers or place addresses in Yonkers!
The GIS Layers script also displays legal addresses, which it retrieves from the above sources. DO NOT rely on this script to set or change address numbers in Yonkers.
You may obtain mailing addresses from any of the sources that are allowable as sources of information for the Waze map, but not from any external copyrighted sources. Waze-provided StreetView images, when the house number is visible, are the best and most reliable source. If no good image is available, then you can get the address from the Yonkers GIS map (see below), a business's own website (but not a third-party copyrighted source), or your own personal "boots on the ground" inspection of the property.
The City of Yonkers maintains its own GIS data with a public map interface, available here. To view mailing addresses, check the "House Numbers" box in the layers panel to the right of the map (it is unchecked by default). At sufficient zoom levels, the house numbers will appear as blue numerals inside a parcel boundary, generally near the edge closest to the street that the address is on. If the association of a number to a particular street is ambiguous, you can verify the address by selecting the "Identify" icon 
in the toolbar at the top of the map, then selecting a blue number; select "House Numbers" on the left panel of the resulting pop-up dialog.
Although the City GIS map is usually reliable, it is not perfect. Direct inspection of the building (either in person or via StreetView), where possible, is the only 100% certain method of verifying the address actually in use.